var arg = {
  "a": '',
};

var c = {"ddd": '78'};
void main() {
  var d;
  //如果被展开的对象可能为 null，需要在展开操作符后面加上 ? 号 (...?)：
  var temp = {...arg, ...c, ...?d};
  var integers = [for (var i = 1; i < 5; i++) i]; // [1, 2, 3, 4]
  var squares = [for (var n in integers) n * n]; // [1, 4, 9, 16]
  var ceshi = [if (3 > 2) '42'];
  print(ceshi);
}
